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April 2024
| DR SAFIAH MA’ALI, SENIOR RESEARCHER: ARC-GRAIN CROPS, POTCHEFSTROOM. FIRST PUBLISHED IN SA GRAAN/GRAIN, SEPTEMBER 2023. |  |
Sunflower oil is one of the major vegetable oils used in the food industry. Moreover, with its high quality, edibility and high protein content, it is often used in the production of various commercial products. Sunflower is generally grown under rain-fed systems due to its moderate drought tolerance.
This drought tolerance can be attributed to its well-developed and deeply penetrating root system that allows the plant to use available soil nutrients and moisture more effectively. However, from the early flowering stage up to the grain-filling stage, sunflower is particularly sensitive to water and heat stress. Environmental factors such as the temperature, day length, intercepted solar radiation and precipitation have varying effects on sunflower growth and developments throughout its lifecycle.
Fluctuations in the temperature and moisture availability affect both the quantity (grain weight) and quality of oil accumulation in sunflower. Therefore, various cultural practices are implemented to counter negative effects. Among these practices, the implementation of a specific planting date and nitrogen (N) fertilisation are some of the most critical factors in successful sunflower production.
NITROGEN FERTILISATION
N is one of the essential nutrients for sunflower growth and development, and constitutes structural and metabolic elements of plant cells, such as amino acids, proteins, nucleic acids and enzymes. Sunflower absorbs N mainly in inorganic forms such as ammonium (NH4+) and nitrate (NO3-).
Upon N absorption, the vegetative growth of plants is stimulated, favouring the synthesis of photo assimilates and by-products for fruit and seed formation. Studies suggested an increase of sunflower seed and oil productivity of up to 40% when supplemented with N sources.
Several international studies reported that a 100 kg N/ha application is optimum for sunflower production. However, the application of N below or above the optimum range reduces the effective use of this nutrient by the crop, which leads to a decline in yield.
The application of excessive N dosages will, for instance, stimulate plant growth (height) to such an extent that the risk of lodging in areas prone to excessive winds increases significantly.
Furthermore, an imbalance between the vegetative and reproductive phases of the crop can occur during these circumstances when uncontrolled vegetative growth is stimulated and plant maturation delayed. This leads to a reduction in yield and an increase in pest susceptibility.
Studies also reported a reduction in oil content and seed yield at a 150 kg N/ha application rate. On the other hand, some studies reported that 80 kg N/ha was sufficient, while others showed that an increase in N levels leads to steady increases in yield, protein contents and linoleic acid simultaneously with a decrease in oil content and oleic acid percentage.
Little or no research has been conducted in South Africa, however, to quantify the optimum N application for local sunflower production conditions. This article will bring some insight on the research currently done by ARC-Grain Crops to address this lack of local information and provide valuable information on N fertilisation and planting date effects on the seed yield and oil content.
PLANTING DATE
The semi-arid Free State and Northwest, with its unpredictable and adverse weather conditions during the production season, account for approximately 80% of the total sunflower cultivation area in South Africa. Consequently, these weather conditions can exert significant influences on the seed yield and oil content.
Late sunflower plantings are also more likely to encounter adverse conditions, including seed or seedling exposure to high December and January temperatures, while pollination and seed development will occur under rapidly decreasing temperatures.
On the other hand, the optimum planting time of this crop allows maximum use of all the natural resources in line with optimal environmental conditions and physiological growth stages of the crop that will ensure good seed germination and seedling establishment, optimum root system development, growth and yield. Plants will be enabled to establish themselves better and absorb essential nutrients from a large volume of soil.
In South Africa, sunflower is typically planted from November to mid-January. If these planting dates are delayed, sunflower grain and oil yields are greatly reduced.
ARC-GRAIN CROPS RESEARCH
Funding from the Oil and Protein Seeds Development Trust and the Agricultural Research Council (ARC) made the ARC-Grain Crops research possible. The aim of the study was to determine the
effect of N fertilisation on sunflower yield when planted at the optimum (mid-December) and late (after mid-January) planting dates. To achieve this, two field experiments were established at ARC-Grain Crops Potchefstroom research farm over two growing seasons (2020/2021 and 2022/2023).
Five sunflower hybrids, including two conventional (PAN 7080 and AGSUN 8251), two Clearfield (AGSUN 5106 CLP and PAN 7160 CLP) and one high oleic hybrid (PAN 7158 HO), were planted to study these effects.
Five different levels of N fertiliser were applied including the control that received no N fertilisation. These included:
1. 45 kg/ha N as a basal application at planting; and
2. 45 kg/ha N one month prior to planting.
This was followed with a topdressing one month after planting applied at:
3. 0 kg/ha.
4. 45 kg/ha (total of 90 kg/ha); and
5. 75 kg/ha (total of 120 kg/ha).
EFFECT OF PLANTING DATE ON SEED YIELD, OIL CONTENT AND OIL YIELD
Table 1 shows the effect of the two planting dates on the sunflower seed yield, oil content and oil yield during these two growing seasons. During the 2021/2022 growing season, the optimum planting date yielded 2,39 t/ha compared to the 1,77 t/ha produced at the late planting date (25% reduction in yield).
The 2022/2023 growing season showed a similar trend, where the first planting date produced 2,30 t/ha, while the late planting produced 1,85 t/ha (19% reduction in yield). These results confirm that of other studies, indicating that early/optimum planted sunflower produced higher yields than those planted later.
The second planting date yielded a 5% (2020/2021) and 4% (2022/2023) higher oil content than that yielded during the optimum planting dates (Table 1). However, the oil yield of the sunflower planted in December (0,93 t/ha and 0,91 t/ha, respectively) was higher than that of the January planting dates (0,73 t/ha and 0,75 t/ha, respectively).
CONCLUSION
DID YOU KNOW?
Click on the link to read the full article, which includes more results about this study.
Publication: April 2024
Section: Pula/Imvula
